The present invention generally relates to a method for filling holes in a substrate and such resulting holes and, more particularly, relates to a method for filling holes in electronic substrates that have high aspect ratios of at least 5:1 in a process for forming vias and interconnects and the resulting holes and vias.
In the electronics packaging industry, there is often a need to fill holes in various substrates for forming vias and interconnects. One such use is for a high performance chip carriers having high small diameter aspect ratio holes. The hole filling process is affected by a number of processing and material parameters, for instance, the diameter of the via hole, the depth of the hole and the type of the substrate material, etc.. Typically, via holes are through holes and are used to electrically connect a top surface and a bottom surface of a substrate and, thus, the filler material utilized in forming the vias must be electrically conductive. A number of techniques have been used to perform the via hole filling process. Depending on the type of the substrate, the techniques may include electroplating, electroless plating, solder paste screen printing and conductive paste screen printing.
A fundamental processing problem arises when the depth-to-width ratio, or the aspect ratio grows. The problem gets more complex when the hole diameter is made aggressively small, i.e., 125 xcexcm or less. For instance, for holes having depth-to-width aspect ratios of 5:1 or larger, and the hole diameter is less than about 125 xcexcm, it is no longer possible to use a conventional technique such as paste screening to fill the holes. Even the more advanced plating techniques cannot be used alone when diameters further decrease and aspect ratios further increase.
An attempt to fill via holes that have 17:1 aspect ratio with conductive epoxies by a screen printing method was found ineffective, even when the screening process is conducted from both the top and the bottom side of the substrate. One other attempt to fill a glass substrate that has via holes of 17:1 aspect ratio was carried out by utilizing a conductive material that has extremely low viscosity, i.e., an eutectic solder that has a viscosity of only 2 centipoise in its molten state, approaching that of water. Even at such low viscosity, known techniques for filling the high aspect ratio via holes were found ineffective. For instance, the conventional techniques tried include an injection molded solder technique with only pressure utilized. The injection molded solder technique was not able to fill deep vias holes that have very small diameters, i.e., smaller than 25 xcexcm.
A second technique of vacuum injection molding was also tried. A graphic illustration of the vacuum injection molding process is shown in FIGS. 1Axcx9c1D. The vacuum injection molding process was disclosed in a co-pending application that was assigned to the common assignee of the present invention under Ser. No. 08/518,874. The vacuum injection molding method utilizes a pressure differential formed between either ambient and vacuum or positive pressure and vacuum. The pressure and the vacuum are both supplied on the same surface of a substrate 10, as shown in FIG. 1A. The process is carried out by utilizing a shallow vacuum link 12 that allows a continual evacuation of air from via holes 20 that have a large aspect ratio such as 5:1. The vacuum link 12 must be sufficiently shallow such that the surface tension of molten solder prevents cross-leaking during the operation. Such a shallow link 12 effectively choke a significant part of the full pressure differential and thus producing only partial filling of via holes 20 that have high aspect ratios.
As shown in FIG. 1A, an injection head 14 which includes spaced apart vacuum slot 16 and injection slot 22 is positioned on top of a mold plate, or substrate 10 in fluid communication with the mold cavities, or via holes 20 contained therein. Relative axial sliding is effected between the injection head 14 and the mold plate 10 for sequentially evacuating gas from the mold cavities 20 using a continuous vacuum and injecting into the evacuated mold cavities 20 a liquid fed from a continuous source (not shown). The sliding of the injection head 14 over the mold plate 10, as shown in FIGS. 1B and 1C, automatically provides self valving for sequentially evacuating and filling the mold cavities 20 from the same side, i.e., the top side 18 of the mold plate 10. In a preferred embodiment, the vacuum slot 16 and the injection slot 22 are linked together at the mold plate 10 so that surface tension of the liquid restrains flow of the liquid (not shown) from the injection slot 22 to the vacuum slot 16 while allowing gas flow thereinbetween for effecting vacuum in the mold cavities 20.
As shown in FIG. 1A, at the start of the process, i.e., before scanning begins, the mold cavities 20 are empty. As the scanning process begins, as shown in FIG. 1B, some cavities 24 are evacuated. As the scanning of the injection head 14 continues, some cavities 26 are filled with the liquid. In the final step of the process, as shown in FIG. 1D, the vacuum supply to the vacuum slot 16 is turned off while the scanning of the injection head 14 is completed, i.e., all the cavities 20, 24 and 26 are filled with liquid.
The vacuum injection molding method illustrated in FIGS. 1Axcx9c1D applies a vacuum and a liquid injection on the same surface of the substrate. To allow the continual evacuation of air from mold cavities that have a larger aspect ratio such as 5:1. The drawback of the process is that the vacuum link must be sufficiently shallow such that the surface tension of molten solder prevents cross-leaking during the operation. The shallow link chokes off a significant part of the full pressure differential and thus only produces partial filling of the mold cavities, or the via holes that have high aspect ratios.
It is an object of the present invention to provide high aspect ratio filled via holes in an electronic substrate and a method of forming certain of these filled holes.
In accordance with the present invention, high aspect ratio filled via holes in electronic substrates are provided, as well as a method of forming certain of these holes.
In a preferred embodiment, an apparatus for filling a liquid in high aspect ratio holes in a substrate is provided which includes a filler plate adapted for receiving an injection head therein, the filler plate has a bottom surface adapted for receiving an injection slot provided on the injection head, a vacuum plate adapted for receiving a vacuum slot in a top surface, the vacuum slot is in fluid communication with a vacuum source, and a connection means for connecting the filler plate and the vacuum plate together in a face-to-face, spaced-apart relationship forming a gap thereinbetween with the bottom surface of the filler plate positioned parallel and opposite to the top surface of the vacuum plate, the connection means further includes an adjustment means for adjusting a relative axial position of the filler plate to the vacuum plate, the gap formed between the filler plate and the vacuum plate is sufficiently large to slidingly engaging a substrate therein and to form abutting contacts between the substrate and the bottom surface of the filler plate and between the substrate and the top surface of the vacuum plate so that holes in the substrate are first evacuated by the vacuum slot and then filled with a liquid by the injection slot in the filler plate.
In the apparatus for filling a liquid in high aspect ratio holes in a substrate, the adjustment means may include a veneer adjustment for fixing a relative axial position of the filler plate to the vacuum plate. The adjustment means may further include a veneer adjustment for transforming a circumferential displacement into a linear axial displacement. The filler plate may be positioned by the adjustment means to lag behind the vacuum plate such that the injection slot lags behind the vacuum slot. The filler plate may be positioned by the adjustment means at the same axial position of the vacuum plate such that the injection slot and the vacuum slot are in fluid communication with a hole in the substrate simultaneously.
In the apparatus for filling a liquid in high aspect ratio holes in a substrate, the injection slot may have a width sufficiently large to cover substantially all the high aspect ratio holes in the substrate. The injection head may be in fluid communication with a liquid reservoir for feeding a liquid through the injection slot. The injection slot may be adapted for feeding a high electrical conductivity liquid solder therethrough. The connection means may further include an adjustment means of a threaded veneer mechanically attached to the filler plate and at least one engagement pin connecting the threaded veneer to the vacuum plate. The high aspect ratio holes in the substrate are selected from the group consisting of via holes, deep through holes and deep trenches.
In the apparatus for filling a liquid in high aspect ratio holes in an electronic substrate, the high aspect ratio holes may have an aspect ratio of at least 5:1, or an aspect ratio of at least 20:1. The high aspect ratio holes may have a diameter not smaller than 5 xcexcm. The electronic substrate may be adapted for making sliding movement between the filler plate and the vacuum plate at a speed of between about 25 mm/min and about 250 mm/min. The apparatus may further include a drive means slidingly engaging the substrate between the filler plate and the vacuum plate.
The present invention is further directed to a method for filling a liquid into holes that have aspect ratios of larger than 5:1 in a substrate which can be carried out by the operating steps of providing a substrate that is equipped with a plurality of through holes that have aspect ratios of larger than 5:1, each of the plurality of through holes has a first opening on a first surface of the substrate and a second opening on an opposite second surface of the substrate, contacting the first surface of the substrate with a vacuum plate equipped with a vacuum slot such that air is evacuated from the first opening of each of the plurality of through holes, and contacting the second surface of the substrate with a filler plate equipped with an injection slot such that a liquid can be injected into the second opening of each of the plurality of through holes which was evacuated by the vacuum plate. The method also includes forming a conductive cap on the filled hole, or forming a conductive lining in each hole, or both.
The method for filling a liquid into holes that have aspect ratios of larger than 5:1 in a substrate may further include the step of contacting the first surface of the substrate with a vacuum plate and contacting the second surface of the substrate with a filler plate simultaneously. The method may further include the step of connecting the vacuum plate to the filler plate by a connection means which further includes an adjustment means of a veneer adjuster. The method may further include the step of adjusting the veneer adjuster such that the injection slot lags behind the vacuum slot when a substrate is slidingly displaced inbetween the filler plate and the vacuum plate. The method may further include the step of slidingly engaging a substrate between the filler plate and the vacuum plate, or slidingly moving a substrate inbetween the filler plate and the vacuum plate by a motor means, or the step of slidingly moving the substrate inbetween the filler plate and the vacuum plate at a speed between about 25 mm/min and about 250 mm/min.
The method for filling a liquid into high aspect ratio holes in a substrate may further include the step of covering substantially all the through holes in the substrate by the injection slots situated in the filler plate when the substrate is placed between the filler plate and the vacuum plate. The method may further include the step of connecting the vacuum plate to the filler plate in such a way that the vacuum plate is situated below the filler plate. The method may further include the step of providing a pressurized liquid reservoir in fluid communication with the injection slot, or the step of feeding a liquid to the injection slot in the filler plate from a liquid reservoir.
The method may further include the steps of feeding a molten solder in a liquid form to the injection slot in the filler plate, injecting the molten solder into the through holes, and solidifying the molten solder and forming via contacts. The method may further include the step of connecting a vacuum evacuation source to the vacuum slot in the vacuum plate such that air is evacuated from the first opening of each of the plurality of through holes.
There is also shown an apparatus for filling an electrically conductive material into high aspect ratio holes in an electronic substrate which includes a first plate equipped with an injection slot for delivering an electrically conductive material, a second plate equipped with a vacuum slot for evacuating air, means for fixing the first and second plates together in a parallely face-to-face, spaced-apart relationship forming a dispensing unit for intimately engaging an electronic substrate therein, the electronic substrate may have holes with aspect ratios larger than 5:1 therein and the holes are in fluid communication with the injection slot and the vacuum slot when intimately engaged between the first and second plates, and drive means for causing sliding engagement and relative motion between the dispensing unit and an electronic substrate positioned therein.
In the apparatus for filling an electrically conductive material into high aspect ratio holes, the apparatus may further include an electrically conductive material reservoir for feeding the material into the injection slot. The apparatus may further include vacuum evacuation means for withdrawing air from the vacuum slot. The fixing means may further include adjustment means for adjusting a relative position of the first plate to the second plate, the adjustment means may be a veneer adjuster. The injection slot in the first plate may be adjusted behind the vacuum slot in the second plate, or the injection slot in the first plate may be adjusted to the same axial position of the vacuum slot in the second plate. The electrically conductive material delivered may be a molten solder at the lower end of the viscosity range, or a conductive polymer at the higher end of the viscosity range. The drive means drives the electronic substrate and cause it to slide between the first and second plates. The drive means may be a motor means.
The invention also contemplates a filled hole having an aspect ratio of greater than 5:1 and a diameter of less than about 125 xcexcm.
Also contemplated is a filled hole which may be plated with a conductor, and having a conductive cap, or both, and a method of forming such a hole.